Git subrepo is a better* way to handle GIT submodules.

*other people recommended it, after struggling a lot with GIT submodules

You have two repos: source_repo and mono_repo.

Clone source_repo into mono_repo:

  cd mono_repo
git subrepo clone `source_repo`

Update mono_repo with new changes

  # still in `mono_repo`

git subrepo pull `source_repo`

Update source_repo with changes made in mono_repo

  # still in `mono_repo`

git add .
git commint -m "made changes including stuff in `source_repo`
git push
# and push to `source_repo`
git subrepo push source_repo

cd source_repo
git pull

This will commit your changes that you did in mono_repo and push it to the source_repo

Again, the command is

git subrepo push source_repo